1. Field of the Invention
This invention relates generally to the structure of high-density optical fiber cables employed in optical communication networks. More particularly, the invention relates to the configuration of grooves in the walls of the core of an optical cable having a plurality of tape-shaped optical fiber bundles therein.
2. Description of the Prior Art
One type of optical fiber cable includes one or more cores, each having grooves therein. Tape-shaped optical fiber bundles are inserted into the grooves. A tape is wrapped around each core after the optical fiber bundles are inserted into the grooves. Then, a sheath is wrapped around all of the taped cores. Dimensions of the grooves have not been previously regarded as important. Therefore, the width of each groove is typically made slightly larger than the width of each tape-shaped optical fiber bundle. The depth of each groove is made to accommodate the thickness of the number of tape-shaped bundles to be placed in the groove one on top of each other.
It has been thought that if the groove dimensions are thus selected to easily accommodate bundles to be inserted therein, that the resulting optical fiber would function well. However, it has been found that performance is affected by groove dimensions. The optical fiber bundle must have sufficient room to make moderate bends as it winds around its core without being so free as to nonuniformal twist the optical fibers of the bundle. If nonuniformal twisting occurs, transmission loss is increased and mechanical strength is decreased. If the groove dimensions are not properly selected, the optical fiber bundles are then subjected to side pressures which cause degraded performance. Also, the lifetime of the cable may be short. However, until now, no particular attention has been paid to the dimensions of the tape-shaped bundles and the grooves which accommodate the bundles in trying to prevent the deterioration of transmission signals and mechanical strength.